Apparatus for electrostatic charging of paper in electrophotographic reproduction

ABSTRACT

A corona discharge electrode is supplied with voltage from a high frequency power source to produce a corona discharge which strikes a sheet of reproducing paper to charge the same electrostatically. A detection electrode is positioned in the field of the corona discharge and is connected via a resistor and amplifier to the power source to control the same in accordance with the detected corona discharge to maintain the magnitude of the discharge field constant. A switch is connected in the power supply to interrupt connection thereof with the discharge electrode if a spark discharge is produced between the discharge electrode and the paper.

United States Patent Ukai 1 Oct. 17, 1972 [54] APPARATUS FORELECTROSTATIC 2,868,989 1/1959 Haacke ..317/262 A CHARGING OF PAPER IN3,390,266 6/1968 Epping ..3l7/262 A ELECTROPHOTOGRAPHIC FOREIGN PATENTSOR APPLICATI NS REPRODUCTION O Inventor: Takeshi Ukai, Tokyo, Japan748,015 12/1966 Canada ..3l7/262A [73] Assignee: Kabushiki Kaisha Ricoh,Tokyo, Primary Examiner-L. T. I-lix Japan Attorney-Waters, Roditi,Schwartz & Nissen [22] Filed: May 17, 1971 [57] ABSTRACT 2 App! 1 a Acorona discharge electrode is supplied with voltage Related A fi fi Datafrom a high frequency power source to produce a corona discharge whichstrikes a sheet of reproducing [63] commuatlonlrlpart of 5 June paper tocharge the same electrostatically. A detection l 1968' electrode ispositioned in the field of the corona discharge and is connected via aresistor and amplifier [30] Fomg Apphcam Pnomy Data to the power sourceto control the same in accordance July 6, 19 7 j n 42/43571 with, thedetected corona discharge to maintain the magnitude of the dischargefield constant. A switch is [52] U.S. Cl ..317/4, 317/262 A nn c in thpower supply to interrupt connection 51 Int. Cl. ..G03g 15/02, HOlt19/00 thereof with the discharge electrode if a spark [58] Field ofSearch ..3 1 7/262 R, 262 A, 4 discharge is produced between thedischarge electrode and the paper. [56] References Cited UNITED STATESPATENTS Larsen et a]. ..317/262 R 5 Claims, 1 Drawing Figure BRIEFSUMMARY OF THE INVENTION The invention relates to apparatus and methodsfor controlling high voltage current supplied to a corona dischargeelectrode in electrophotographic reproduction apparatus. 7

Heretofore it has been difficult to stabilize the operation of a coronadischarge electrode as its charging ac- .tion is substantially affectedby changes in power voltage, loads or atmospheric conditions.

An object of the present invention is to provide a charging deviceincorporating means for stabilizing the charging action and obviate anyvariations thereof.

According to the invention a corona current detecting means isincorporated in the charging device for controlling the change of coronacurrent for eliminating instability and for maintaining a uniform that auniform copy may be obtained.

According to a specific embodiment of the invention there is provided acharge control circuit for electrophotographic reproducing apparatuswhich comprises a high voltage power source, a corona dischargeelectrode electrically connected to said source for producing a coronadischarge current, a target plate for supporting a sheet of reproducingpaper, said target plate being grounded and facing said dischargeelectrode to expose the sheet of reproducing paper to thecoronadischarge current from said corona discharge electrode wherebysaid sheet is electrostatically charged, a detection electrodepositioned adjacent said discharge electrode to detect corona dischargecurrent from said discharge electrode, and feedback means connected tosaid discharge electrode and to said high charge so voltage power sourceto control said power source in accordance with the detected coronadischarge current to maintain the value of said current constant.

The discharge electrode and detection electrode are placed adjacent one,another in a hollow, grounded guide member which has an opening facingthe target plate. Thus, the discharge between the discharge electrodeand the detection electrode is substantially constant regardless of thepresence or absence of a copy sheet on the target plate.

A further object of the invention is to provide a safety device which isoperative to interrupt supply of power to the discharge electrode if aspark discharge should be produced between the discharge electrode andthe paper due to contact therebetween, or the like, which could producea fire.

In further accordance with the invention the safety device is operativewhen the detection electrode detects the absence of a corona dischargefield upon the production of a spark discharge between the dischargeelectrode and the copy paper.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram of thecharge control apparatus according to the invention; and

FIG. 2 is a perspective view, partly broken away, of a portion of theapparatus of FIG. 1.

DETAILED DESCRIPTION Referring to the drawing, therein is seen a coronadischarge electrode 1 constituted as a tungsten wire. Mounted adjacentelectrode 1 is a detection electrode 2 also in the form of a tungstenwire. The detection electrode 2 is intended to detect the magnitude ofthe corona discharge field developed by the electrode 1. The electrodes1 and 2 are mounted in a hollow guide member 3. which is connected toground and is pro vided with an opening facing a grounded target plate10 on which a sheet of reproducing paper 11 can be placed. The hollowmember 3 servesto confine the corona discharge and direct the dischargeto the paper 11 to charge the paper electrostatically. The intensity ofthe corona field is the same at the detection electrode 2 as at theplate 10 with the exception that the electrode 2 is not subject to thevariable effects of the paper 11 as is the plate 10. Thus, the currentdetected in the electrode 2 is an accurate representation of theintensity of the corona field produced by the electrode 1. As will beseen hereafter the current in the electrode 2 will serve as a means tocontrol the voltage applied to electrode 1 in order to stabilize thecorona discharge and render the charge on the copy paper 1 l uniform.

The detection electrode 2 is connected to a variable resistor 4 acrosswhich a voltage is produced related to the current in detectionelectrode 2. The resistor 4 is connected to the input of an amplifier 5which produces an amplified output signal of the voltage in resistor 4.The output of the amplifier produced at the plate of a triode V isconnected to the control grid of an oscillating vacuum tube 6 whichserves to control the voltage input to the discharge electrode 1 byvirtue of its connection via resonance condenser 7 to the primarywinding of a high frequency transformer 8. The secondary of thetransformer is connected to a high voltage rectifier 9 which isconnectedto discharge electrode l.

In an oscillation circuit consisting of the oscillating vacuum tube 6,the resonance condenser 7 and the transformer 8, when resonance currentis caused to flow in the primary winding of the transformer 8 throughits output, sine wave high voltage is simultaneously induced in thesecondary winding of the transformer 8. This sine wave high voltage ischanged to negative high voltage by the high voltage rectifier 9 andapplied to the discharge electrode 1. Thereby, a corona field isproducedand corona current flows between the discharge electrode 1, thedetection electrode 2, the corona guide member 3 and the paper 11. Thecorona current which flows from the electrode 1 to the paper 11 isapproximately proportional to the corona current which flows from theelectrode 1 to ground through the detection electrode 2 and the variableresistor 4.

When the negative corona current which flows from the dischargeelectrode 1 to the paper 11 is increased excessively due to changes ofpower voltage, changes in the paper and atmospheric conditions, thenegative corona current which flows from the electrode 1 to groundthrough electrode 2 and the variable resistor 4 is also increased andthe electric potential at the point flowing from the electrode 1 isdecreased, a similar ac-.

tion is produced, namely, the corona current is increased contrary tothe above-mentioned case and the balance is maintained. In this way, thecorona current is always kept at a fixed value, and the chargingoperation is stabilized withoutany unevenness so that a uniform copy canbe obtained by the electrophotographic reproduction apparatus. Thus, theresistor 4, amplifier 5 and oscillation circuit constitute a feedbackmeansto control the power source of transformer 8 and rectifier 9 tomaintain the value of the corona discharge current constant.

The same object is obtainable by providing the oscillation circuit ofthe invention with a pulse generator, thereby changing-the pulseinterval by using the detected voltage of the variable resistor 4 andcontrolling the high voltage applied to the discharge electrode 1. Thedetection electrode 2 need not be arranged adjacent to the electrode 1and for example, a shielding box (similar to corona guide member 3) canbe employed for electrode 1, but the shielding box is normally groundeddirectly. Preferably, however, the electrodes l and 2 are mounted inhollow guide member 3 with the electrode 2 remote from paper 11 andspaced further therefrom than is the electrode 1, in order to eliminateany effect of the paper 11 on the current detected in detectionelectrode 2.

From the above it is seen that a corona current detection produces asignal which is employed as a feedback to the corona generating powersupply to maintain the corona current constant without any change sothat a uniform copy may be obtained.

The circuit of the invention is also constructed to eliminate thepossibility of a fire in the event that the copy paper should contactthe discharge electrode 1 to produce a spark discharge therebetween.

Accordingly, a safety device is provided which detects the presence of aspark discharge to open a switch 40 connected between the primarywinding of trans former 8 and the voltage source. As seen in the circuitin FIG. 1, a grid of a triode V is connected to the plate of triode V,through a Zener diode Zn. A coil of a holding type relay 40' isconnected to the plate of triode V Any holding type relay can be used ifthe open state of its contacts is maintained until the relay onceoperated is reset by hand.

A time constant circuit comprising a diode 30, a resistor 31, acondenser 32 and a resistor 33 is connected between the plate of thetriode V and the Zener diode Zn. If a button is pushed to supply thecharging device with power, a discharge will be effected in thedischarge electrode 1 after a short delay of time during which theheater of the vacuum tube is heated. Herein, if there is not providedthis time lag mechanism, the triode V is operated to open the switchcontacts 40 of the relay 40'. In the state of normal use, the potentialof the plate of the triode V, is lower than that of the Zener diode andthe triode V is cut off. If a sheet of paper is caught in the chargingdevice and a spark discharge is produced, the discharge between thedischarge electrode 1 and the detecting electrode 2 will be interruptedand the voltage of the resistor 4 will become zero. With the increase ofthe potential of the grid of the triode V the'potential of the plate ofthe triode V will be lowered, and the triode V will be cut off and thepotential of its plate will be increased, the Zener diode Zn will beoperated,

the triode V will become conductive, and the normally closed switchcontacts 40 of the relay coil 40' will be opened. These contacts can beclosed by resetting them by hand after the trouble has been corrected.

Although the invention has been described with reference to a specificembodiment thereof, the invention is not limited thereto andmodifications and variations will become evident to those skilled in theart without departure from the spirit and scope of the invention asdefined in the appended claims.

What is claimed is:

l. A charge control circuit for electrophotographic reproducingapparatus, said charge control circuit comprising a high voltage powersource, a corona discharge electrode electrically connected to said.source for producing a corona discharge current, a tar-.

get plate for supporting a sheet of reproducing paper, said target platebeing grounded and facing said discharge electrode to expose the sheetof reproducing paper to the corona discharge current from said coronadischarge electrode whereby said sheet is electrostatically charged, adetection electrode positioned adjacent said discharge electrode todetect corona discharge current from said discharge electrode, feedbackmeans connected to said discharge electrode and to said high voltagepower source to control said power source in accordance with thedetected corona discharge current to maintain the value of said currentconstant, and means connected to said power source to interrupt theconnection thereof to said discharge electrode upon elimination ofdischarge current betweenthe discharge electrode and the detectionelectrode when a spark discharge is produced between the dischargeelectrode and the reproducing paper, said power. source comprising ahigh voltage transfonner with primary and secondary windings, and a highvoltage rectifier connected to the secondary winding of the transformerand to the discharge electrode, said means to interrupt the connectionof the power source to the discharge electrode comprising a switch meansfor interrupting power supply to the primary winding of the transformer,a relay controlling said switch means, and means including a timercircuit connected to said feedback means and to said relay to operatesaid relay and open said switch means if said feedback means detectslack of corona discharge current.

2. A circuit as claimed in claim 1 comprising a hollow grounded guidemember having an opening facing said target plate, said dischargeelectrode and detection electrode being adjacent one another within saidguide member.

3. A circuit as claimed in claim 2 wherein said discharge electrode iscloser to said target plate than is said detection electrode.

4. A circuit as claimed in claim 1 wherein said feedback means comprisesa variable resistor connected to said detection electrode, amplifiermeans having an input connected to said variable resistor, control meansconnected to the amplifier means at the output thereof and to the powersource to adjust the latter in accurrent to render said triodeconductive whereupon the relay is activated and said switch means isopened.

5. A circuit as claimed in claim 4 wherein said timer circuit comprisesa diode, a resistor means and a condenser connected between said Zenerdiode and said amplifier means.

1. A charge control circuit for electrophotographic reproducingapparatus, said charge control circuit comprising a high voltage powersource, a corona discharge electrode electrically connected to saidsource for producing a corona discharge current, a target plate forsupporting a sheet of reproducing paper, said target plate beinggrounded and facing said discharge electrode to expose the sheet ofreproducing paper to the corona discharge current from said coronadischarge electrode whereby said sheet is electrostatically charged, adetection electrode positioned adjacent said discharge electrode todetect corona discharge current from said discharge electrode, feedbackmeans connected to said discharge electrode and to said high voltagepower source to control said power source in accordance with thedetected corona discharge current to maintain the value of said currentconstant, and means connected to said power source to interrupt theconnection thereof to said discharge electrode upon elimination ofdischarge current between the discharge electrode and the detectionelectrode when a spark discharge is produced between the dischargeelectrode and the reproducing paper, said power source comprising a highvoltage transformer with primary and secondary windings, and a highvoltage rectifier connected to the secondary winding of the transformerand to the discharge electrode, said means to interrupt the connectionof the power source to the discharge electrode comprising a switch meansfor interrupting power supply to the primary winding of the transformer,a relay controlling said switch means, and means including a timercircuit connected to said feedback means and to said relay to operatesaid relay and open said switch means if said feedback means detectslack of corona discharge current.
 2. A circuit as claimed in claim 1comprising a hollow grounded guide member having an opening facing saidtarget plate, said discharge electrode and detection electrode beingadjacent one another within said guide member.
 3. A circuit as claimedin claim 2 wherein said discharge electrode is closer to said targetplate than is said detection electrode.
 4. A circuit as claimed in claim1 wherein said feedback means comprises a variable resistor connected tosaid detection electrode, amplifier means having an input connected tosaid variable resistor, control means connected to the amplifier meansat the output thereof and to the power source to adjust the latter inaccordance with the variations produced by the output signal from theamplifier means, said means for operating the relay comprising a triodeconnected to said relay, a Zener diode connected between said triode andsaid amplifier means and normally inoperative when said variableresistor is subjected to corona discharge current, said Zener diodebeing operative when the variable resistor is not subjected to coronadischarge current to render said triode conductive whereupon the relayis activated and said switch means is opened.
 5. A circuit as claimed inclaim 4 wherein said timer circuit comprises a diode, a resistor meansand a condenser connected between said Zener diode and said amplifiermeans.